A diagonal alone does not determine screen width and height. The aspect ratio supplies the shape, while the diagonal supplies the physical scale.

Reduce the pixel ratio

Divide width and height by their greatest common divisor. 2560×1440 reduces to 16:9. The unreduced numbers remain necessary for PPI, while 16:9 is convenient for describing shape.

Scale width and height

After calculating the pixel diagonal, physical width = diagonal × pixel width ÷ pixel diagonal; height uses the same formula. A 27-inch 2560×1440 panel is about 23.532540×13.237053 inches.

Use the visible active area

Bezels and housing are outside the panel rectangle. If you measure hardware, measure the active image diagonal rather than the outside case, and expect small differences from rounded advertised sizes.